The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing CO and HC emissions is an oxidation catalyst device (“OC”). The OC device includes a flow-through substrate and a catalyst compound applied to the substrate. The catalyst compound of the OC induces an oxidation reaction of the exhaust gases once the OC device has attained a threshold or light-off temperature. One type of exhaust treatment technology for reducing NOx emissions is a selective catalytic reduction (“SCR”) device that may be positioned downstream of the OC device. The SCR device includes a substrate, having a SCR catalyst compound applied to the substrate. A reductant is typically sprayed into hot exhaust gases upstream of the SCR device. The reductant may be a urea solution that decomposes to ammonia (“NH3”) in the hot exhaust gases and is adsorbed by the SCR device. The ammonia then reduces the NOx to nitrogen in the presence of the SCR catalyst. The SCR device needs to reach a threshold or light-off temperature to effectively reduce the NOx to nitrogen. Before reaching the threshold or light-off temperature, the SCR acts as a NOx adsorber in the presence of stored NH3. Specifically, the SCR device traps NOx and stores the NOx by forming intermediates such as ammonium nitrates and/or nitrites with NH3 until the light-off temperature has been reached. Once the light-off temperature is attained, the NOx is reduced to nitrogen in the presence of the SCR catalyst.
In addition to the gaseous emissions emitted from an internal combustion engine, the exhaust gas also contains water vapor. Specifically, for example, the exhaust gas of a diesel engine typically contains between about 0.5 to about 8 percent water vapor. However, water vapor in the exhaust gas may decrease the NOx adsorption capacity over the SCR catalyst compound before reaching the light-off temperature. Accordingly, it is desirable to provide an efficient approach for limiting the amount of water vapor in the presence of the SCR device before the SCR device reaches the light-off temperature.